1. Field of the Invention
The present invention relates to a radio frequency coil for use with a magnetic resonance (MR) imaging device and, more particularly, to a spiral volume radio frequency coil which eliminates much or all of the spatial variation in the intensity and contrast of MR images caused by the short wavelengths in high static magnetic fields without compromising performance.
2. Description of the Prior Art
Higher static magnetic field strength can increase the sensitivity of magnetic resonance imaging and spectroscopy studies. As noted by D. I. Hoult et al. in an article entitled: "The Sensitivity of the Zeugmatographic Experiment Involving Human Samples," J. Magn. Reson., Vol. 34, pp. 425-433 (1979), the signal-to-noise ratio of a study increases roughly linearly with magnetic field strength for large samples and even faster for small ones. As noted by Ogawa et al. in an article entitled: "Oxygenation-sensitive contrast in magnetic resonance imaging of rodent brain at high magnetic fields," Magn. Reson. Med., Vol. 14, pp. 268-78 (1990), image contrast can also be increased, especially with susceptibility based contrast mechanisms such as blood oxygenation contrast. Spectroscopy studies may benefit from greater spectral resolution because T.sub.2 changes only slowly with frequency. Despite these advantages, a number of technical obstacles have impeded the progress of high field MR in human subjects. Though static field inhomogeneity, longer T.sub.1, and power deposition are important issues, the primary obstacle has been radio frequency (RF) coil technology.
At frequencies above 100 MHZ, the decreasing RF wavelength becomes a serious problem. Even in the absence of a sample, problems with phase shifts, parasitic capacitance and radiative losses complicate the construction of RF coils. Fortunately, recent work by J. T. Vaughan et al. in an article entitled: "High Frequency Volume Coils For Clinical NMR Imaging and Spectroscopy," Magn. Reson. Med., Vol. 32, pp. 206-218 (1994), and by H. Wen et al. in an article entitled: "The Design and Test of a New Volume Coil for High Field Imaging," Magn. Reson. Med., Vol. 32, pp. 492-498 (1994), has demonstrated that distributed capacitance, careful design and the use of RF shields can overcome these coil construction problems. Most high field volume coil designs have attempted to achieve the low frequency ideal of a sinusoidally distributed current with negligible phase shift along the axis of the coil. Imaging studies and simulations have shown, however, that this type of coil produces higher RF field in the center of the sample than towards the outside. With this degree of RF inhomogeneity, the sensitivity at the outside of the sample may not be substantially improved over lower field images with better homogeneity. Contrast and image intensity with many pulse sequences will also vary with position, making them less easily interpretable. Often high field imaging results have employed special pulse sequences of the type disclosed by Vaughan et al. in the afore-mentioned article, by Ugurbil et al. in an article entitled: "Imaging at High Magnetic Fields: Initial Experiences at 4T," Magnetic Resonance Quarterly, Vol. 9, pp. 259-277 (1993), and by J. H. Lee et al. in an article entitled: "High Contrast and Fast Three-Dimensional Magnetic Resonance Imaging at High Fields," Magn. Reson. Med., Vol. 34, pp. 308-312 (1995), which are chosen to minimize intensity variations from RF inhomogeneity.
As noted by Bottomley et al. in an article entitled: "RF Magnetic Field Penetration, Phase Shift and Power Dissipation in Biological Tissue: Implications for NMR Imaging," Physics in Medicine and Biology, Vol. 23, pp. 630-643 (1978), two factors lead to decreased RF field homogeneity within samples at high field strength. One factor is the shorter penetration distance, or skin depth, of RF into the sample because of electrical conductivity. This skin depth decreases approximately as the square root of the frequency, or field strength. As the skin depth decreases, the field should become weaker at the center of the sample. The second factor affecting field homogeneity is the short wavelength of light in the sample. Because of the high dielectric constant of water and tissue at these frequencies, the wavelength of light is almost eight times shorter in the sample than in air. This short wavelength implies that the amplitude or phase of the RF must vary with position inside the sample. The center brightening of high field images is a natural consequence of this requirement for spatial variation of RF.
While spatial variation of the amplitude of the RF field complicates MR studies, spatial variation of the phase of the RF is relatively benign. As long as the phase variation within an imaging or spectroscopic voxel, typically much smaller than the sample, is much less than 180.degree., the results will be unaffected. A coil designed to produce a spatially varying RF phase but a spatially uniform RF magnitude could provide a practical solution to the problem of high field RF inhomogeneity. In an article entitled: "Reduction of RF Penetration Effects in High Field Imaging," Magn. Reson. Med., Vol. 23, pp. 287-301 (1992), Foo et al. first emphasized the value of a cylindrical coil which produces a linear phase variation along its axis for improved RF homogeneity at high field. Foo et al. were able to produce this phase variation in a coil by driving a traveling wave from one end of a dielectric filled coil and absorbing it resistively at the other. The absorption was necessary because reflection of the traveling waves would cause standing wave patterns with very poor axial field homogeneity. Though improved spatial homogeneity of the RF was observed, the resistive termination made the coil extremely inefficient and unacceptable for practical use.
A previous attempt to improve the spatial uniformity of high field imaging was published in Magnetic Resonance in Medicine, Vol. 23, pp. 287-301 (1992). In that article, it was recognized that a volume coil which produced a linear phase variation along its axis would improve the image uniformity. However, no practical coil to achieve this was described.
Other coils for high field strength head imaging have been reported in Magn. Reson. in Med., Vol. 32, pp. 206-218 and pp. 492-498 (1994). However, neither of the described coils attempts to improve the image uniformity. In addition, a coil that produces a linear phase variation was constructed and reported in Magn. Reson. Med., Vol. 23, pp. 287-301 (1992), but the coil used dielectrics and resistive terminators in a way that reduce the S/N ratio by orders of magnitude. Images were never acquired with the coil as a result.
Finite wavelength effects in the sample cause the RF produced by standard cylindrical MR volume coils to be greater in magnitude at the center than towards the outside of the sample. For many pulse sequences, the inhomogeneity of the RF field leads to center brightening of images as well as large variations of signal to noise ratio and contrast throughout the image. Since the wave equation requires that there be spatial variation of the RF field, inhomogeneity of the RF field is unavoidable. However, spatial variation can be present without affecting most MR images if the magnitude of the RF field is constant while only the RF phase varies with position. This principle was emphasized by Foo et al., who recognized that a cylindrical volume coil that generated a linear phase shift along its axis could eliminate the magnitude inhomogeneity in the RF field. However, to the inventor's knowledge, a practical coil for achieving this field distribution has not yet been presented.
A spiral coil has been used for a different purpose, namely, for tuning the coil simultaneously to two frequencies. However, the use of a spiral coil to produce a linear phase variation along its axis in high magnetic field strengths is not known to the inventor.
Accordingly, it is known in the art that increased magnetic field strength improves the sensitivity and some forms of contrast in MRI. As the field is increased, the frequency of the MR signal increases proportionally. At field strengths above 1.5 Tesla, the wavelength of the radio frequency signal inside of biological samples becomes shorter than the size of the human head and body. Such short wavelengths tend to cause variations with position in the intensity and contrast of the MR images. In many parts of the head or body, the improved image quality of high field is not realized because of these spatial variations. A high RF frequency field coil design that addresses these problems and realizes a linear phase variation along its axis while preserving efficiency and field homogeneity along its axis is not known in the art. The present invention has been developed to address this need in the art.